The present invention relates to a non-volatile semiconductor memory device and its data programming method.
A memory cell of an NAND type EEPROM is shown in FIG. 28. FIG. 28 is a schematic sectional view of the memory cell MC. The gate insulation file GO which is formed between floating gate FG and channel region CA is sufficiently thin to allow an electron tunnel effect to occur between the floating gate FG and the channel region CA. Electrons are emitted from the floating gate FG to the channel region CA by supplying a high voltage to the substrate Sb, and 0 V to the control gate CG, so that one of a binary data is stored in the memory cell. On the other hand, when the other of a binary data is stored in the memory cell, electrons are injected from the channel region CA to the floating gate FG. In other to inject electrons to the floating gate FG, a high voltage is supplied to the control gate CG, and 0 V is supplied to the channel region CA, source S and drain D. A part of a memory cell array of the NAND type EEPROM is shown in FIG. 29(a). The memory cells MC are serially connected between the select transistor ST and the transistor 10. Further, FIG. 29(b) shows voltage waveform diagrams at the respective nodes shown in FIG. 29(a). When data are programmed to the memory cells MC, first electrons are emitted from the floating gates FG of all the memory cells MC to the substrate by setting all the row lines WL1 to WLn connected to the control gates CG to 0 V and the substrate to a high voltage. After that, a high voltage is supplied to the row select line S connected to the gates of the select transistors ST connected to the memory cells MC to which data are to be written. At the same time, signal xcfx86 which is applied to the gates of the transistors 10 is set to 0 V to turn off transistors 10 to disconnect the memory cells MC from the reference potential VSS. When electrons are injected to the floating gates FG of the memory cells MC, the row line WL corresponding thereto is set to the high potential V1, and column lines D corresponding thereto are set to 0 V. At this time, a potential difference between the floating gates FG and the channel region CA becomes large enough to occur the electron tunnel effect, so electrons are injected to the floating gates FG from the channel region CA. On the other hand, the non-selected row lines WL are set to the potential V2 lower than the high potential V1. At this time, although the potential of the column lines D is 0 V, since the potential V2 is low, the potential difference between the floating gates FG of the memory cells applied the potential V2 and the channel region CA is not large enough to cause the electron tunnel effect, so electrons are not injected to the floating gates FG of the memory cells applied the potential V2. In the memory cells connected to the row line WL of the high potential V1, if the column lines D are set to the potential V3, since the potential difference between the floating gates FG and the channel region CA is not large enough to cause the electron tunnel effect, electrons are not injected to the floating gates FG. In FIG. 29(b), at time T1, electrons are injected to the floating gate FG of memory cell 2n, but not injected into the floating gate FG of the memory cell 1n because the voltage of the column line D1 is the potential V3. In the same way, at time T2, electrons are injected to the floating gate FG of memory cell 11, but not injected to the floating gate FG of the memory cell 21.
In the memory cells MC, if the electrons are injected to the floating gate FG thereof, the threshold voltage thereof becomes a positive value; and if the electrons are emitted from the floating gate FG thereof, the threshold voltage thereof becomes a negative value. In a data read mode, when the memory cell MC is selected, the gate thereof is set to a logic xe2x80x9c0xe2x80x9d, for instance to 0 V. When the threshold voltage of the selected memory cell MC is the negative value, the selected memory cell MC is turned on. However, when the threshold voltage of the selected memory cell MC is the positive value, the selected memory cell MC is turned off. The data stored in the selected memory cell MC is detected depending on whether or not the selected memory cell MC is turned on. On the other hand, the gate of the non-selected memory cell MC connected to the selected memory cell is set to a logic xe2x80x9c1xe2x80x9d, for instance to 5 V. So the non-selected memory cell MC is turned on even when electrons are injected into the floating gate.
Referring to FIG. 30, the data reading operation will be explained. The depletion type MOS transistor L1 which acts as a load for the memory cell, the select transistor (enhancement type) ST, the memory cells M1 to M8, and the transistor 10 are connected in series between the power supply voltage VDD and the reference potential (VSS). The gate of the transistor L1 is connected to a connecting point (node N1) between the transistor L1 and the select transistor ST. The signal X for selecting memory block 11 composed of the memory cells M1 to M8 is supplied to the gate of the select transistor ST. Further, the signals W1 to W8 for selecting one of the memory cell of the memory block 11 are supplied to the gates of the memory cells M1 to M8, respectively. The sense amplifier 12 is connected to the node N1. The data stored in the selected memory cell can be read by being detected the voltage level of the node N1 by the sense amplifier 12. In the data read node, the signal xcfx86 which is applied to the gate of the transistor 10 is a logic xe2x80x9c1xe2x80x9d. So the transistor 10 is turned on. The memory block 11 is connected to the reference potential through the transistor 10 in the data read mode. In the circuit shown in FIG. 30, the assumption is made that electrons are emitted from the floating gates of the memory cells M2 and M4, so the threshold voltages of the memory cells M2 and M4 are negative, and further the memory cell transistor M4 is selected, for instance. The threshold voltages of the other memory cells M1, M3, and M5 to M8 are positive. In this case, as shown by a timing chart in FIG. 31, the signal X is set to a logic xe2x80x9c1xe2x80x9d, the signals W1 to W3, W5 to W8 are set to a logic xe2x80x9c1xe2x80x9d, and the signal W4 is set to a logic xe2x80x9c0xe2x80x9d. By the above-mentioned setting, the select transistor ST and the memory cells M1 to M3 and M5 to M8 are turned on. Further, since the threshold voltage of the memory cell M4 is negative, this memory cell M4 is also turned on. Accordingly, the node N1 is discharged toward the reference potential through the select transistor ST, the memory cells M1 to M8, and the transistor 10. The data stored in the memory cell M4 can be read by being detected the voltage of the node N1 by the sense amplifier 12. After that, the memory cell M3 is assumed to be selected. In this case, the signal W3 is set to a logic xe2x80x9c0xe2x80x9d, and all other signals are set to a logic xe2x80x9c1xe2x80x9d. In this case, since the threshold voltage of the memory cell M3 is positive, the memory cell M3 is turned off. Thus, since the discharging path of the node N1 toward the reference potential is cut off, this node N1 is charged toward the power source voltage VDD through the transistor L1. By detecting the charged voltage of the node N1, the data can be read from the memory cell M3.
However, in the case where data of a logic xe2x80x9c1xe2x80x9d or a logic xe2x80x9c0xe2x80x9d is stored in the memory cell depending on whether the threshold voltage of the memory cell is negative or positive, the amount of current flowing through the memory block is according to the number of the memory cells having positive threshold voltages and the number of the memory cells having negative threshold voltages which are included in the memory block, so the amount of current flowing through each of the memory blocks is different from each other. Thus, the discharging speed at the node N1 differs according to the number of the memory cells whose threshold voltages are positive and the number of the memory cells whose threshold voltages are negative in the series-connected memory cells of the memory block.
For instance, as shown in FIG. 32(a), in the case where electrons are injected to the floating gates of the memory cells M1 to M7 so that these memory cells have a positive threshold voltage and further where only the memory cell M8 has a negative threshold voltage. If the memory cell M8 is selected, since the threshold voltages of all the other transistors M1 to M7 are positive, the amount of the current flowing through the memory block 11 is a minimum. On the other hand, as shown in FIG. 32(b), in the case where all the memory cells M1 to M8 for constituting the memory block 11 have a negative threshold voltage, the amount of the current flowing through the memory block 11 is a maximum. Since the potentials of the signals W1 to W8 shown in FIG. 32(b) are the same as those shown in FIG. 32(a), a larger current flows through the memory cells whose threshold voltages are negative, compared with the memory cells whose threshold voltages are positive. Accordingly, in the NAND type EEPROM, there exist shortcomings in that the data read speed is determined depending on the ratio of the number of the memory cells having the positive threshold voltage to the number of the memory cells having the negative threshold voltage in the memory block, as described above. In addition, since the current flowing through the memory block of FIG. 32(a) is the minimum, it is necessary to determine the current driving capability of the load transistor L1 in accordance with the minimum current of the memory block, for this reason it is impossible to increase the current driving capability of the load transistor L1. Consequently, the charging speed at the node N1 is also lowered.
Furthermore, in the above-mentioned conventional memory device, one memory cell block corresponds to one column line. So, the area occupied by the memory cell array has been determined the junction portions between the memory cell blocks and the column lines at the two adjacent memory cell blocks. Further, the production yield has been influenced by an increase in the number of the column line wires and the number of the junction portions.
Further, one of a binary data are written by emitting electrons from the memory cells of all the memory cells at the same time to set the threshold voltages of the memory cells to the negative value. After that, the other of a binary data are written by selectively injecting electrons to the floating gates thereof.
When data are read from the memory cells, the selected row line is set to a logic xe2x80x9c0xe2x80x9d (e.g., 0 V) and the non-selected row lines are set to a logic xe2x80x9c1xe2x80x9d (e.g., 5 V). Since the non-selected row lines are at a logic xe2x80x9c1xe2x80x9d, the non-selected memory cells are turned on irrespective of whether the threshold voltages thereof are positive or not. On the other hand, the selected row line is at 0 V. Accordingly, if the threshold voltage of the selected memory cell is positive, the selected memory cell is turned off, and if the threshold voltage of the selected memory cell is negative, the selected memory cell is turned on. As already described above, the data stored in the memory cell is detected depending on whether the selected memory cell is turned on or off. The memory cell which has a positive threshold voltage is turned on if a logic xe2x80x9c1xe2x80x9d level signal is applied to the control gate thereof, and is turned off if a logic xe2x80x9c0xe2x80x9d level signal is applied to the control gate thereof. Therefore, care must be taken about the quantity of electrons injected into the floating gate. Accordingly, the injection of electrons and the reading for checking the quantity of electrons injected into the floating gate are carried out repeatedly, and the injection of electrons is stopped whenever the threshold voltage of the memory cell reaches an appropriate value. However, since electrons are injected through an extremely thin gate insulation film, the threshold voltages of the memory cells after the injecting of electrons do not become uniform and vary according to a certain distribution owing to subtle variations in the quality of the gate insulation film and a manufacturing process. Thus, the threshold voltages of the memory cells in which electrons are injected are distributed within a range. Accordingly, a difference in the threshold voltage between the memory cell having the maximum threshold voltage and that having the minimum threshold voltage causes a difference in current flowing through the memory cell between the two, so that the data reading speeds from the selected memory cells differ according to the threshold voltages of the memory cells. In other words, since the current flows through the selected memory cell and the non-selected memory cells connected to the selected memory cell in series, the distribution of the threshold voltages of the non-selected memory cells causes directly the dispersion in the current flowing through the non-selected memory cells, so that the data read speed distributes according to the non-selected memory cells connected to the selected memory cell. To get a high data read speed, it is preferable to flow a large current through the memory cell. However, the threshold voltages of the memory cells in which electrons are injected must have a positive value. So, even if the threshold voltage of the memory cell having a minimum value is set to a value slightly higher than 0 V, the threshold voltage of the memory cell having a maximum value is inevitably a value far higher than 0 V due to the distribution of the threshold voltages of the memory cells in which electrons are injected.
In addition, in conventional NOR type flash EEPROM, when data is programmed, first, electrons are injected to the floating gates of all the memory cells to uniformalize the quantities of electrons stored in the floating gates of all the memory cells, and then electrons are emitted from the floating gates of all the memory cells. The other of a binary data is selectively written in the selected memory cell by applying a high voltage to the control gate and the drain of the selected memory cell to flow a channel current and thereby to inject electrons from the channel region to the floating gate of the selected memory cell. In the conventional NOR type flash EEPROM as described above, however, when electrons are emitted from the floating gate of the memory cell excessively, the threshold voltage of the memory cell becomes a negative value, so each non-selected memory cell whose gate voltage is set to 0 V is rendered conductive, disabling a selecting operation. In the conventional NOR type flash EEPROM, therefore, the emitting of electrons and the reading of data of the memory cell are performed repeatedly, and the electron-emitted state of the floating gate is checked in each read operation so that the emitting of electrons is stopped when the threshold voltage of the memory cell reaches a proper value. To prevent electrons from being emitted excessively, a period of the emitting electrons is set to be short, and the emitting and the reading are repeated several times to obtain an appropriate threshold voltage. In this emitting of electrons, electrons are emitted from the floating gate to the source or drain of the memory cell by the electron tunnel effect, by applying 0 V to the control gate and a high voltage to the source or drain thereof. Accordingly, the thickness of the gate insulation film between the floating gate and the channel region is formed with an extremely, for example about 100 angstroms, so that the electron tunnel effect can be obtained. As a result, the threshold voltages of the memory cells obtained after electrons have been emitted are not uniform and thereby distribute within a range in all the memory cells, due to subtle variations in the manufacturing process. When the current flowing through the memory cell is large, the high data read speed is got. Accordingly, it is preferable that the threshold voltage of the memory cell is low. However, when electrons are emitted until an optimal threshold voltage can be obtained in the memory cell having the highest threshold voltage within the distribution, the threshold voltage of the memory cell having the lowest threshold voltage within the distribution becomes a negative value. To avoid this problem, in the EEPROM as described above, the threshold voltage of the memory cell having the lowest threshold voltage within the distribution is determined so as to become a positive value. Consequently, the data read speed of the memory cell having the highest threshold voltage within the distribution is delayed, and thereby it has been difficult to get a high data read speed.
FIGS. 33(a) to (d) show an example of the memory cell array of the conventional NOR type flash EEPROM, in which FIG. 33(a) is a plan view; FIG. 33(b) is a cross-sectional view taken along the line A-Axe2x80x2; FIG. 33(c) is a cross-sectional view taken along the line B-Bxe2x80x2; and FIG. 33(d) is a cross-sectional view taken along the line C-Cxe2x80x2. Further, FIG. 34 is a symbolic diagram thereof. In FIGS. 33(a) to (d), numeral 1 shows the row lines which form control gates of the memory cells. Numeral 2 shows the floating gates; 3 shows the channel regions; and 4 shows the gate insulation films. Numeral 5 shows the column lines formed of aluminum, for instance, which are connected to the drains 6 used in common for the two adjacent memory cells. Numeral 8 shows a wire formed of aluminum, for instance, for supplying a reference potential (e.g., a ground potential) when data are read and a high voltage when electrons are emitted from the floating gates, which is connected to source 7 of the memory cell at junction point 9 so as to be used in common for the two adjacent memory cells.
In the conventional nor type flash EEPROM constructed as described above, when electrons are emitted excessively from the floating gate and thereby the threshold voltage of the memory cell becomes a negative value, the non-selected memory cells whose control gates are at 0 V are turned on. Consequently, the column line 5 and the wire 8 are connected through the non-selected memory cell, so that current flows from the column line 5 to the wire 8 through the non-selected memory cell. Thus, when data are read or written, even if a voltage is applied to the column line 5, since current flows through the non-selected memory cell, the applied voltage drops. Accordingly, even if the selected memory cell is turned off in the data read operation, since current flows through the non-selected memory cell, erroneous data are to be read and further it is impossible to supply a necessary and sufficient voltage in data writing. As a result, as already explained, in the dispersion range of all the memory cells after electrons have been emitted from the floating gates, since it is necessary to set the threshold voltage of the memory cell having the lowest threshold voltage to a positive value, the data read speed is determined by that of the memory cell having the highest threshold voltage, with the result that it is impossible to get a high data read speed.
As already explained, in the EEPROM, data are programmed by injecting and emitting electrons to and from the floating gates through the oxide film with a thickness of about 100 angstroms which is far thinner than that of the gate oxide film. FIG. 35 is a symbolic diagram showing a memory cell of a conventional EEPROM for programming data in further a different way. Here, the control gate voltage is showed by VCG; the drain voltage is showed by VD; the source voltage is showed by VS; and the drain current is showed by ID, the drain current ID relative to the control gate voltage VCG can be represented by the characteristics as shown in FIG. 36. In FIG. 36, curve A represents the initial characteristics; curve B represents the characteristics when electrons are injected to the floating gate, in which the threshold voltage is raised due to the injection of electrons; and curve C represents the characteristics when electrons are emitted from the floating gate, in which the threshold voltage change to negative due to the emission of electrons. In the memory cell, data of a logic xe2x80x9c0xe2x80x9d and a logic xe2x80x9c1xe2x80x9d are stored by use of the characteristics represented by the curves B and C.
FIG. 37 shows an example of the circuit configuration of the EEPROM constructed by arranging the memory cells as shown in FIG. 35 into a matrix pattern. As shown in FIG. 37, selecting MOS transistor ST is connected in series to memory cell transistor CT, and one memory cell 14 is composed of two transistors CT and ST. In the configuration as described above, when electrons are injected to the floating gate of the memory cell transistor CT, high voltages VG and VC0 are applied to the gate of the selecting transistor ST and the control gate of the memory cell transistor CT, respectively, and in addition the column line 15 is set to 0 V. On the other hand, when electrons are emitted from the floating gate, the gate of the selecting transistor ST and the column line 15 are set to high voltages and the control gate of the memory cell transistor CT is set to 0 V. Thus, the high voltage is applied to the drain of the memory cell transistor CT, so that electrons are emitted from the floating gate to the drain.
FIG. 38(a) is a pattern plan view of region 16 enclosed by dot-dashed lines in the circuit shown in FIG. 37, and FIG. 38(a) shows a cross section taken along the line A-Axe2x80x2 in FIG. 38(a). In both FIGS. 38(a) and (b), the same reference numerals have been retained for the parts or elements corresponding to those shown in FIG. 37, and further the numeral 17 shows the source region of the memory cell transistor CT; 18 shows the drain of the memory cell transistor CT and the source region of the selecting transistor ST; 19 shows the drain region of the selecting transistor ST; 20 shows the floating gate of the memory cell transistor CT; 21 shows the control gate of the memory cell transistor CT; 22 shows the gate of the selecting transistor ST; 23 shows a thin gate insulation film; and 24 shows a contact portion between the column line 15 and the drain of the selecting transistor ST.
In the EEPROM as described above, in order to shorten the time required to program the data to the memory cells, as shown in FIG. 39, latch circuits L are provided for each column line 15. Further, data to be programmed to the memory cells 14 connected to the respective corresponding column line 15 are first latched by the latch circuits L respectively, and data are simultaneously programmed to the memory cells 14 for one row in accordance with the latched data of the latch circuit. In the EEPROM as constructed above, although it is possible to shorten the time required to program the data, since the latch circuits are provided for each column line, there exists a drawback in that the chip size increases to the extent required for the latch circuits, and thereby the chip cost increases.
In the semiconductor memory device already explained by FIGS. 28 to 32, the current flowing through the memory block differs according to the ratio of the number of the memory cells (constituting the memory block) having the positive threshold voltages to that having the negative threshold voltages. Accordingly, when the memory cells of the memory block include many MOS transistors having the positive voltages, the data read speed is delayed. In addition, since the current flowing through the memory block of FIG. 32(a) is the minimum, it is necessary to determine the current driving capability of the load transistor L1 in accordance with the minimum current of the memory block, for this reason it is impossible to increase the current driving capability of the load transistor L1. Consequently, the charging speed at the node N1 is also lowered.
With the above described situation in mind, therefore, it is an object of the first aspect of the present invention to provide a non-volatile semiconductor memory device in which large current can be flowed through the memory block to improve the data read speed.
Furthermore, as already described, in the conventional semiconductor memory device, since one column line is formed in correspondence to the memory block of one column, there exist various problems.
With the above described situation in mind, therefore, another object of the second aspect of the present invention is to provide a program method for realizing a non-volatile semiconductor memory, by which the number of the column lines can be reduced to one half by using one column line in common for two adjacent memory cell blocks, to allow the area occupied by the memory cell array not be decided by the junction portions between the column line and each of the two adjacent memory cell blocks.
Further, in the conventional non-volatile semiconductor memory cell already described on the basis of FIGS. 28 and 29, there exists a drawback in that the electron injection rate to the floating gate of the memory cell disperses due to the dispersion in thickness of the gate insulation film of the memory cell and crystal defects caused by the dispersion during the manufacturing process.
With the above described situation in mind, therefore, another object of the third aspect of the present invention is to provide a non-volatile semiconductor memory by which the dispersion in the threshold voltage of the memory cell in which electrons are injected to the floating gate can be reduced.
Furthermore, in the conventional NOR type flash EEPROM described with reference to FIGS. 33 and 34, among the memory cells in which electrons are emitted from the floating gates, since it is necessary to set the threshold voltage of the memory cell having the lowest threshold voltage to a positive value, the data read speed is decided by the memory cell having the highest threshold voltage, so that there exists a drawback in that it is impossible to increase the data read speed.
With the above described situation in mind, therefore, another object of fourth aspect of the present invention is to provide a non-volatile semiconductor memory cell higher in data read speed.
Further, in the EEPROM as described with reference to FIGS. 35 to 39, as already stated, since the latch circuit is provided for each column line, the chip size increases to the extent of the latch circuits, so that there exists a drawback in that the chip coat is high.
With the above described situation in mind, therefore, another object of the fifth aspect of the present invention is to provide a non-volatile semiconductor memory which is low in cost and short in program time in the same way as with the case of the conventional memory.
To achieve the above stated object, the first aspect of the present invention provides a non-volatile semiconductor memory, comprising: a memory cell block having a plurality of memory cells each formed as a transistor having a floating gate being connected in series, each memory cell storing one or the other data of binary data on the basis of a first status where electrons are injected to the floating gate or a second status where electrons are emitted from the floating gate; a selecting transistor connected in series to one end of said memory cell block, for selecting said memory cell block; switching means connected to the other end of said memory cell block; and a bit checking transistor provided in said memory cell block end connected in series to the memory cells, for deciding a logic status of each memory cell in which electrons are injected to the floating gate and each memory cell in which electrons are emitted from the floating gates in the memory cell block.
Furthermore, the second aspect of the present invention provides a data programming method for programming data to memory cells in a non-volatile semiconductor memory having:
a plurality of memory cell blocks each having a plurality of memory cells each formed as a transistor having a floating gate, said memory cells being connected in series; a plurality of selecting transistors each connected in series to one end of each of the memory cell blocks, for selecting each memory cell block; first row lines each connected to the memory cells on the same row in a plurality of memory blocks arranged in a matrix pattern, each of said memory blocks being composed of the memory cell block and the selecting transistor; column lines each connected to at least two adjacent memory blocks; a second row line connected to the selecting transistor of one memory block of the two adjacent memory blocks; and a third row line connected to the selecting transistor of the other memory block of the two adjacent memory block; and a plurality of switching means each connected between the other end of each of said memory cell blocks and a reference potential, which method comprises the steps of:
turning off a plurality of said switching means; setting the selected first row line to a first potential; setting the non-selected first row lines to a second potential lower than the first potential; setting the second and third row lines to the first potential and setting the column line to a third potential lower than the first potential, to charge the memory cell block beginning from the third potential; turning off the non-selected transistor of the selecting transistors connected to the second and third row lines; and setting the column line to the third potential or 0 V according to data to be programmed to the memory cell.
Further, the third aspect of the present invention provides a non-volatile semiconductor memory cell, comprising: a plurality of memory cell blocks having a plurality of memory cells each formed as a transistor having a floating gate and a control gate connected in series; a plurality of selecting transistors each connected in series to one end of each of said memory cell blocks, for selecting each of said memory cell blocks; row lines each connected to the control gates of the memory cells arranged in the same row of the matrix-arranged selecting transistors and the memory cells; programming means for programming data to the memory cell by applying a program voltage to the row line to inject electrons to the floating gate thereof; and program voltage supplying means for repeatedly checking electron injections and electron injection rates in sequence during programming, by applying a programming voltage higher than the preceding programming voltage to the row line.
Further, the fourth aspect of the present invention provides a non-volatile semiconductor memory, comprising: row lines; a plurality of memory cells each selectively driven by each of said row lines and each formed as a transistor having a floating gate and a control gate respectively, for storing either of binary data on the basis of an electron status in the floating gate thereof; column lines to each of which one end of each of said corresponding memory cells is connected; a plurality of transistors each connected between the other end of each of said memory cells and a reference potential, the gate of each of said transistors being connected to each of said row lines; data erasing means for emitting electrons from the floating gate of each of said memory cells; first writing means for storing one bit data of binary data by injecting electrons to the floating gate of said memory cell in such a way that said memory cell is turned on when selected and off when not selected; and second writing means for storing the other bit data of binary data by injecting electrons to the floating gate of said memory cell in such a way that said memory cell is turned off both when selected and not selected.
Furthermore, the fifth aspect of the present invention provides a non-volatile semiconductor memory, comprising: row lines; memory cells each selectively driven by each of said row lines; column lines connected to each of said memory cells; a column decoder; a plurality of column gate transistors one end of each of which is connected to each of said column lines, for selecting one of said column lines under switching control by said column decoder; and data programming means connected to the other end of each of said column gate transistors, for programming data to said memory cells, by turning on one of said column gate transistors to supply potential to one of said column lines according to data to be programmed to said memory cells, turning off the column gate transistor and holding the applied potential at the column line to program data to said memory cell on the basis of the held potential.
In the first aspect of the present invention, in the each memory cell block, the logic status of the memory cells can be decided by the bit checking transistor on the basis of the memory cells in which electrons have been injected to or emitted from the floating gates.
Therefore, in the memory cells constituting the memory cell block, the number of the memory cells which store the xe2x80x9c0xe2x80x9d data is compared with the number of the memory cells which store the xe2x80x9c1xe2x80x9d data, for instance; and the memory cells storing the more bit data are determined so as to have a negative threshold voltage, and the memory cells storing the less bit data are determined so as to have a positive threshold voltage. The determined threshold voltages of the memory cells can be stored in the bit checking transistors. In other words, with respect to the respective memory cell block, it is possible to determined the threshold voltages of the memory cells in such a way that the number of the memory cells of a negative threshold voltage becomes more than that of the memory cells of a positive threshold voltage, with the result that current flowing through the memory cell block can be increased and thereby the read speed can be improved.
In the second aspect of the present invention, the two adjacent memory cell blocks are connected in common by a single column line. The non-selected memory cell block is charged beginning from a potential lower than the first potential in such a way that the electron tunnel effect will not be caused between the floating gates of the memory cells and the substrate. After that, the selecting transistors of the non-selected memory cell blocks are turned off so that the charged potential can be held in the non-selected memory cell blocks. Accordingly, it is possible to use in common a single column line for the two adjacent memory cell blocks. In other words, since the number of the column lines can be reduced half, it is possible to reduce the ratio of an area occupied by the junction portions between the column line and each of the two adjacent memory cell blocks, to the area occupied by the memory cell array.
In the third aspect of the present invention, when data are programmed to the memory cells, a program voltage is applied to the row line. In this program, the electron injection and emission to and from the floating gates are repeatedly checked in sequence. Here, the program voltage higher than before is applied one after another. By doing this, irrespective of the dispersion during the manufacturing process, electrons are injected to the respective memory cells under optimal conditions, thus reducing the dispersion of the threshold voltages of the memory cells.
In the fourth aspect of the present invention, after electrons have been once emitted from the floating gates of the memory cells to lower the threshold voltages to a negative value, electrons are injected to the floating gates of the memory cells in such a way that the memory cells are turned on when selected and off when not selected through the first writing means, so that one of a binary data can be stored in the memory cells.
After that, electrons are selectively injected to the floating gates of the memory cells in such a way that the memory cells are turned on both when selected and not selected through the second writing means, so that the other of a binary data can be stored.
In the fifth aspect of the present invention, the column lines are connected to the data programming means through the column gate transistors. By the column decoder, the column gate transistors are selectively turned on. After that, the column line is set to a potential according to data to be programmed by the data programming means. The memory cells can be programmed on the basis of the potentials. In the program potentials, current hardly flows and thereby power is hardly consumed. Consequently, it is possible to form the data latch circuits of the data programming means at any positions freely remote from the memory cell array.